1. Field of the Invention (Technical Field):
The present invention relates to methods and apparatuses for generating magnetic and/or electric fields with highly uniform field strengths and directions (dipole fields), highly uniform radial gradients of the field strengths within the body of the device (quadrupole fields), or higher order fields (sextupole, octupole, and above) within the body of the device that are highly uniform in the desired radial field derivative. The invention also relates to associated methods and apparatuses for effective cancellation of fields external to the device, and for rotation of fields in space.
The uniform magnetic field and field cancellation devices apply most dramatically to medical magnetic resonance imaging (MRI) and other imaging applications, such as nuclear magnetic resonance microscopy and spectroscopy, and to charged particle beam guiding.
Uniform electric fields have application in charged particle beam guiding, for example in electron beam devices such as oscilloscopes and in mass spectrometers and charged particle energy analyzers.
Uniform dipole fields, uniform radial magnetic field gradients, and higher order multipole magnets have application in charged particle beam guiding and focusing for ion implantation, electron beam devices and in high-power charged particle accelerators.
Uniform and higher order electric field gradients have application in the separation of components of liquids or gases with different dielectric constants, or separation of components with different electric dipole moments whether electric field induced or permanent, such as metal particles in a gas or fluid, or water dispersed in oil.
2. Background Art:
Background Art in the Generation of Uniform (Dipole) Magnetic Fields:
Present devices for generating uniform magnetic fields in relatively large volumes are of three basic types: resistive solenoid magnets; superconductive solenoid magnets; and permanent magnets. Each device has significant drawbacks. Conventional resistive magnets are handicapped by limited field strength (approximately 0.2 T), cooling requirements, power consumption of 50 kW or more, high inductance which makes pulsed operation impractical, generation of substantial fringe fields, and poor patient access in MRI applications.
Conventional superconductive magnets, while providing for high fields (0.5 T to 2 T), have the disadvantages of high cost, need for complex cryogenic systems that are expensive to operate, high inductance (cannot be pulsed), generation of substantial fringe fields, and poor patient access.
Permanent magnets have lower fringe fields and good patient access, but have low magnetic fields (less than 0.1 T), are not adjustable in field strength, cannot be pulsed, and are very heavy (typically more than 12,000 pounds for a 0.064 T system).
An extremely important use of uniform magnetic field generation is MRI diagnostic procedures. They have the critical advantage of being non-invasive, and are not known to cause biological damage. MRI systems employ a strong constant uniform magnetic field (usually 0.3 T to 1.5 T) to align the magnetic dipoles of proton nuclear spins. These aligned dipoles are then tipped out of alignment by a radio frequency pulse. The constant applied field attempts to force the spinning dipoles back into alignment and they precess around the field direction, much like a gyroscope. This coherent precession and spin relaxation produces a radiated signal that is analyzed to produce an image. The actual process is more complicated, using field gradients and a variety of signal processing methods. In all systems, the image quality depends critically on the homogeneity and stability of the applied magnetic field.
The disadvantages of present MRI systems center primarily on the high-field magnet required and its external effects. MRI systems are expensive, with a high-field (1.5 T) system costing about $1,500,000. Much of the total cost of the facility, however, is due to site requirements pertaining to effects of the magnet's field on external objects and the effects of those objects on the magnet. The magnetic fields of conventional MRI systems extend far from the magnet. Referring to FIG. 1, in the conventional solenoidal coil magnet the current-carrying conductors wrap around the axis and the magnetic field lines must close on the outside of the magnet. This is true even if external cancellation coils or shields are used. Ferromagnetic objects near the magnet are strongly attracted toward the bore of the magnet, causing a missile hazard, and also perturb the magnetic field and the concomitant imaging process. The fields can affect pacemakers, instruments, computers, video devices, watches, and even electric motors sufficiently close to the magnet. The Food and Drug Administration (FDA) requires that human access be restricted in areas in which the magnetic field exceeds 0.0005 T (5 G). FIG. 2, taken from Partain et al., Magnetic Resonance Imaging vol. 2 (2d ed. 1988), shows the range of the field and recommended minimum distances for various items from a conventional 0.5 T magnet. These requirements are such that the MRI device often cannot be located in a hospital, but must be in a separate remote site. These costs are necessarily reflected in cost to the patient.
The conventional solenoidal magnet configuration also has other disadvantages for the patient. The imaging processes presently in use require the patient to lie as still as possible in the magnet, which is enclosed except for one end, for 30 minutes or more. While every effort is made to reduce the feeling of confinement, 5% to 10% of patients are unable to undergo the procedure because of claustrophobic effects. The procedure is especially difficult for children, particularly because the FDA mandated restricted access inside the 5 G boundary generally excludes everyone but the patient from the room. In addition, access to the patient is difficult, making it hard to ascertain patient status, position, or movement.
The present invention provides devices and methods for generating uniform magnetic fields that reduce or eliminate the disadvantages of present systems listed above. The invention uses a relatively small number of conductors parallel to the long axis, preferably connecting together at the ends. The patient, in the preferred embodiment, enters through the side in a natural opening, rather than through an end, and is not in an enclosed space. The patient may be oriented either along the axis or across the magnet (see FIG. 3). Most importantly, with the use of the field canceling system, the external fields decrease very rapidly with distance from the magnet, permitting elimination of shielding. FIGS. 4a and 4b show field lines in a 16-wire embodiment of the present invention without (FIG. 4a) and with (FIG. 4b) field canceling coils. The field lines extend well outside the magnet without the field canceling coils, but are confined inside the magnet with field canceling coils. Furthermore, the inductance of the present invention is much lower than that of conventional solenoidal magnets, permitting pulsing of the system.
Background Art in the Generation of Uniform Quadrupole, Sextupole, and Higher Order Magnetic Fields:
Present devices for generating uniform quadrupole (uniform radial gradient), sextupole (uniform first derivative of the radial gradient), and higher order magnetic fields are generally of three types: those that use appropriately shaped iron or a similar ferromagnetic material wound with current-carrying coils; systems of current-carrying conductors without iron cores; and shaped permanent magnet assemblies. These magnetic devices are used primarily for focusing and guiding charged particle beams.
The most common configurations use iron cores and coils as shown in FIG. 5. These devices have the following disadvantages: (1) the useful volume of the device is a small fraction of the total because the beam must be inside the pole faces and most of the volume is occupied by the coils and iron cores; (2) the iron cores are a complex shape and must be machined with high accuracy; (3) the accuracy of the system is limited by the accuracy of the machining of the pole faces and by small variations in the homogeneity of the permeability of the iron core; and (4) the maximum magnetic field strength available in this device is limited by onset of saturation of the iron core, which introduces nonuniformities in the fields and field gradients.
Systems using permanent magnets in lieu of current-carrying coils have most of the same limitations listed above, in that they usually use shaped iron pole faces or shaped permanent magnets. In addition, such systems are limited in field strength by the permanent magnets and are not readily adjustable in field strength.
Systems using coils without iron cores avoid many of the above difficulties, but generally depend on approximations to a sin(.phi.) or cos(.phi.) current distribution of limited accuracy, depend on shims and wedges to improve accuracies of the fields, and have a useful beam area that is small relative to the magnet size. FIG. 6 shows a schematic of a dipole magnet in use in several accelerators. Schmuser, "Superconducting Magnets for Particle Accelerators," AIP Conference Proceedings 249, vol. 2, Physics of Particle Accelerators (American Institute of Physics, 1992).
The present invention, in the configuration that produces uniform field gradients or higher order multipoles, eliminates the need for iron cores to achieve high accuracy, is simple in construction in that it requires a relatively small number of conductors to achieve high accuracy in a volume that is a high percentage of the total device volume, and is not limited in field strength by saturation of iron cores. The system is well-behaved, and can readily achieve field gradient accuracies much greater than devices using iron cores. The present invention also has the advantages of open construction and easy access. This is advantageous when employing, e.g., beam test instrumentation or other beam guiding elements, such as electrical guiding plates.
The present invention, in the quadrupole and higher order configurations, can also use a system of field cancellation coils similar to those of the uniform field configuration to reduce stray external fields to a very low value. Field cancellation in the radial gradient (quadrupole) and higher order multipole configurations is even more effective than in the dipole field configuration, since the main generating coils are quadrupole and higher order, rather than dipole, and so the external fields generated by the combination of the main and field canceling coils behave as even higher order multipoles, and decrease more rapidly with distance.
Background Art in the Generation of Uniform Electric Fields:
Electric fields that are uniform in magnitude and direction are commonly generated by establishing an electric potential difference between two parallel conducting plates. Until the conception of the present invention there has been no method for locating individual charged line conductors in a configuration that produces a volume of uniform electric field that is large compared to the volume of the apparatus. In the present invention, in the configuration of linear conductors or loops that produce uniform magnetic fields when energized with a current, if the conductors each have electric charges of identical magnitudes applied (opposite polarity in the conductors that had opposite currents in the magnetic field configuration of the invention), the result is an electric field that has the same uniformity characteristics as the magnetic field configuration of the invention, but rotated ninety degrees.
Uniform electric fields have applications in guiding and turning charged particle beams, for example electron beams in oscilloscopes and similar fluorescent-screen imaging devices, in high energy particle accelerators, and in charged-particle-beam energy analyzers.
This linear-conductor large-volume electric dipole has the advantages of being light in weight and open for access to the useful center area. It also has very low capacitance, since the conductor surface area is very low compared to the parallel-plate devices, and so the generated electric field can be ramped at very high rates, which is useful for swept beams in imaging devices and time-resolved energy analyzers. In beam turning applications, a small difference in the charges applied to the conductors in each of the two halves of the device results in a small linear electric field gradient in the beam region, which is useful for beam centering in the turning system.
Leakage fields external to the device can be greatly decreased without solid shielding in the same way as with the magnetic field device. A system of conductors with the same azimuthal locations as the main field generating conductors, but at larger radius and carrying charges that are opposite in polarity and reduced in magnitude from the main conductor charges by the ratio of the main conductor cylinder radius to the field cancellation conductor cylinder radius reduces the external fields in the same manner as in the magnetic field version of the device.
Background Art in the Generation of Rotating Fields:
Highly uniform rotating magnetic fields have particular application in nuclear magnetic resonance spectroscopy, where at present solid samples are sometimes rotated at high speeds with rotation axes at or near a particular angle (about 54 degrees, the so-called "magic angle") with the magnetic field, which significantly enhances signal-to-noise ratios. The present invention allows high-speed rotation of the magnetic field, keeping the sample stationary. This permits rotation angular velocities far in excess of that achievable by mechanical systems, allowing extension to new regimes of rotation velocities, and to samples that would be destroyed when mechanically rotated in existing devices. The system of conductors that produces the rotating transverse magnetic field can be combined with an appropriately designed solenoid that produces a uniform constant longitudinal field so that the rotating net field vector makes any desired angle with the longitudinal axis of the system. Rotating fields can also be applied to imaging techniques, limited in the case of medical imaging to maximum rates of change of magnetic field set by the U.S. Food and Drug Administration (FDA).